Alkali metals in combustion of biomass with coal

Growing demand for energy in the world, depletion of fossil fuels and green house effect require from us to utilize alternative, renewable sources of power. Biomass gained in the last few years more and more attention especially in Europe. Many research programs focused on the various forms of thermal biomass utilization have been launched and successfully accomplished expanding our knowledge and contributing to the, so-called, sustainable development. Utilization of straw, biomass present in Europe in large although spread quantities, is an interesting option among others for small decentralized CHP plants. On the other side, straw thermal utilization can cause serious problems resulting in power plant shut downs. The chemical composition of straw, especially high volatile alkali metals content in combination with other elements like chlorine causes corrosion and deposits formation problems, moreover, in combination with silica and calcium slagging and fouling problems. The main goal of this thesis was to investigate the mechanisms responsible for alkali metals release and sequestering during combustion of straw and the influence of co-combustion of straw with coal. The knowledge regarding these mechanisms is necessary to operate biomass fired power plants in a safe, efficient and profitable way. The research has been done by means of experiments and system modeling. The tests have been done using pilot scale CFB combustor and bench scale heated grid reactor together with the fundamental studies over KCl-kaolin interactions in TG reactor. The high alkali (HIAL) straws selected for the experiments were characterized by a broad range of potassium contents, from average values to extremely high potassium content. This in combination with certain ratios of Cl and Si would lead to corrosion and deposit formation problems mentioned above. Finding a way to capture alkali metals by additives in combustion systems, circulating fluidized bed in particular, is the next issue this thesis was aiming at. The screening of possible alkali metals sorbing additives was presented. Further more fundamental investigation of the most promising additive, aluminasilicate clay - kaolin, a natural constituent of coal ash, were shown and novel results were presented